1. Field of the Invention
The present invention relates to a method and an apparatus for surface configuration measurement using a moiré method for detecting the surface defects of a work, the bulging or waving of a ridge, dints or similar irregularity of the kind varying little in tridimensional configuration.
2. Description of the Background Art
Japanese Patent Laid-Open Publication Nos. 2-201142 and 4-169840, for example, each disclose a defect testing method relating to a photoconductive drum or similar cylindrical work. In Laid-Open Publication No. 2-201142, a laser beam issuing from a light source scans the surface of a photoconductive drum via a polygonal mirror in the axial direction of the drum. A photoconductive layer forming the surface of the drum reflects the laser beam. The reflection from the normal surface of the drum is substantially incident to a sensor. The sensor determines the intensity of the incident reflection and feeds its output to a data processing section. The method determines that the surface configuration of the drum is defective when the output of the sensor is unusually low.
On the other hand, in Laid-Open Publication No. 4-169840, a projector includes a halogen lamp or similar light source for issuing light toward a photoconductive drum via a slit. Reflection from the drum and diffused by the defects of the drum is focused by a lens on a line sensor. This method detects unusual reflection ascribable to defects.
A photoconductive drum is likely to have a first kind of defects including pin holes, dents, scratches, bubbles, cracks, and the deposition of dust, which noticeably vary in height. Further, the drum is likely to have a second kind defects including irregularity in the thickness of a photoconductive layer and the drop of a liquid, which vary little in height, as well as the scratches of a support, which do not appear on the surface of the drum. Laid-Open Publications described above can effectively deal with the first kind of defects, but lack accuracy when it comes to the second kind of defects.
A moiré method using moiré topography belongs to a family of tridimensional measuring methods. The moiré method is available in two different types, i.e., a stereoscopic lattice type and a lattice projection type, as will be described specifically later. The moiré method allows a work to be intuitively grasped. However, it is difficult to determine a projection or a recess with the moiré method. Also, the moiré method is not feasible for high-sensitivity tridimensional measurement; the state-of-the-art distance between moiré fringe contour lines is about 10 μm. Further, visibility differs from one moiré fringe to another moiré fringe, so that a moiré image cannot be easily processed.
More specifically, the lattice projection type of moiré method uses two lattices. One of the two lattices is moved to shift the phase of moiré fringes for thereby finely dividing the interval between contour lines in the equivalent sense. Also, this type of moiré method can promote the accurate detection of a projection or a recess as well as sensitivity. By contrast, the stereoscopic lattice type of moiré method, which uses a single lattice, cannot shift the phase while matching the phases of the contour lines of all orders even if the phase shift of the lattice projection type of moiré method is executed.
In light of the above, Japanese Patent Laid-Open No. 4-147001 (Japanese Patent No. 2,887,517), for example, proposes a measuring method that is the combination of the above two different types of moiré methods. In the proposed method, a lattice surface is moved vertically while, at the same time, a light source or a point of observation is moved horizontally. This allows the fringe phase to be shifted relative to a work with fringes of all orders being substantially matched in phase. Therefore, a plurality of moiré fringe images can be processed by the principle of a high-speed scanning method (phase shifting method). This increases the density of points of measurement and allows one period of moiré fringes to be physically divided by about 1/40 to 1/100. The method can therefore implement the detection of a projection or a recess and high sensitivity, which have been difficult to achieve with the stereoscopic lattice type of moiré method.
The method of Laid-Open Publication No. 4-147001, however, contemplates to substantially accurately shift all fringe orders in relation to the application of the phase shifting method to the stereoscopic lattice type of moiré method. As a result, sophisticated mechanisms are essential for the vertical movement of the lattice surface and the horizontal movement of the light source of the point of observation. Moreover, the method must repeatedly shoot a work three times or more, resulting in a long measuring time.
On the other hand, Japanese Patent Laid-Open Publication No. 7-332956 proposes to obviate differences between fringe intervals dependent on fringe order by matching the phases of all fringes at the time of shift. Also, this document implements phase shift based only on lattice movement. However, when a cylindrical work, for example, should have its entire surface measured, this kind of scheme must repeat lattice movement and image pickup while causing the work to make three or more rotations, also resulting in a long measuring time.
Japanese Patent Laid-Open Publication No. 10-54711 teaches a method that shifts the phase by varying the height of a work. This, however, also needs the movement of the work and repeated image pickup, increasing the measuring time. In addition, the above document does not describe any specific procedure for the quantization of the configuration of a projection or that of a recess.